Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Segment	Sep. 30, 2013	Dec. 31, 2013
Segment Information [Abstract]
Number of reportable operating segments defined by geographical regions			3
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	$ 268,118	$ 152,177	$ 683,749	$ 446,017
Net operating income	180,450	101,855	460,887	301,074
Management and other fees	2,361	1,771	6,856	5,812
Depreciation	(102,184)	(48,227)	(254,211)	(142,687)
General and administrative	(11,479)	(6,263)	(28,621)	(19,852)
Merger expenses	(3,857)	0	(46,413)	0
Acquisition and disposition costs	(51)	(237)	(1,555)	(792)
Interest expense	(45,830)	(29,192)	(117,021)	(86,661)
Interest and other income	2,992	2,387	8,685	9,326
Equity income from co-investments	4,910	40,802	21,065	52,295
Gain (loss) on early retirement of debt	0	(178)	0	846
Gains on sale of real estate and land	31,372	0	39,640	1,503
Income from continuing operations	58,684	62,718	89,312	120,864
Net reportable operating segment - real estate assets	9,803,759		9,803,759		4,188,871
Real estate under development	363,193		363,193		50,430
Co-investments	1,043,277		1,043,277		677,133
Real Estate Held-for-sale	107,772		107,772		0
Cash and cash equivalents, including restricted cash	88,000		88,000		53,766
Marketable securities	108,147		108,147		90,084
Notes and other receivables	22,973		22,973		68,255
Other non-segment assets	129,441		129,441		58,300
Total assets	11,666,562		11,666,562		5,186,839
Other Real Estate Assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	8,167	4,971	22,716	14,319
Net operating income	5,723	3,105	14,906	9,742
Net reportable operating segment - real estate assets	147,317		147,317		86,745
Reportable Geographical Components [Member] | Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	118,242	65,805	302,380	195,272
Net operating income	76,725	43,237	198,873	130,398
Net reportable operating segment - real estate assets	4,323,148		4,323,148		1,746,434
Reportable Geographical Components [Member] | Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	94,643	54,189	238,564	156,983
Net operating income	66,287	37,466	166,983	108,481
Net reportable operating segment - real estate assets	3,698,020		3,698,020		1,614,159
Reportable Geographical Components [Member] | Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	47,066	27,212	120,089	79,443
Net operating income	31,715	18,047	80,125	52,453
Net reportable operating segment - real estate assets	$ 1,635,274		$ 1,635,274		$ 741,533